Engines of Our Ingenuity

No. 2387

by Andrew Boyd

Today, just one word. The University of Houstonís College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The 1967 film The Graduate was filled with unforgettable lines. One of the most memorable was advice offered to a young Dustin Hoffman, who was trying to decide what to do with his life.

Adult offering advice: I just want to say one word to you — just one word —
Hoffman: Yes sir.
Adult offering advice: Are you listening?
Hoffman: Yes sir, I am.
Adult offering advice: Plastics.

Today weíre surrounded by plastics. We couldnít live without them. And theyíre so cheap we use them to make disposable products — water bottles, picnic cups, grocery bags, milk jugs — the list goes on and on. We throw away so much plastic that many cities have recycling programs. But recycling plastic isnít as simple as just melting it down and forming it into something new. There are many types of plastics. You canít simply mix one with another and expect to get something usable.

Thatís why the Society of the Plastics Industry developed plastic identification codes. Most plastic containers fall into one of six categories. Each category has a name and a number. Itís the number that shows up on containers, surrounded by three arrows forming a triangle. In most cases youíll find it near the bottom center of a container. Using identification codes was voluntary when they were first introduced. But today most states require it. The codes make it much easier to sort plastics.

labels for the identification of seven different types of plastics

The seven different identification codes for plastics. Code seven is used when nothing else fits.

But just because plastics are sorted properly doesnít mean they can be recycled. Some types are economical to recycle. Others arenít. Plastic number one — polyethylene terephthalate — is easy to recycle. Water and soda bottles are commonly made from it. Plastic number two — high density polyethylene — is also pretty good. Itís found in milk and laundry detergent jugs, and itís often recycled into plastic lumber and made into decks or park benches.

Plastic number five — polypropylene — isnít so easy to recycle. Itís used for yoghurt and margarine tubs because it handles grease and chemicals so well. Disposable dishes are made of plastic number six — polystyrene. Itís also used to make Styrofoam products like packing peanuts and insulated cups. The city I live in recycles number one and two plastics. But weíre asked to sort out and throw away the other numbers. Thereís no recycler the city can sell them to. They go to the landfill with all the other waste. Plastic manufacturers and consumers are aware that some plastics are harder to recycle than others. But solving the problemís not as easy as using only recyclable plastics. Different plastics have different properties. The plastic used to make a soda bottle may not be good for making forks and knives.

So engineers keep looking for improved plastics — plastics that do their job and arenít bad for the environment. Today, we might want to give not one, but two words of advice to the graduate: better plastics.

Iím Andy Boyd at the University of Houston, where weíre interested in the way inventive minds work.

(Theme music)

For related episodes, see New Technologies and Packaging and Waste.


Plastic Recycling. Accessed June 17, 2008, from Wikipedia.

Society for the Plastics Industry Resin Identification Code Guide to Correct Use. Accessed June 17, 2008.

Photos courtesy of Wikipedia.

The following material is taken from the Wikipedia web page Plastic Recycling. Accessed June 17, 2008, from Wikipedia.

Plastic Identification Code

Type of Plastic Polymer


Common Packaging Applications


Polyethylene Terephthalate

Clarity, strength, toughness, barrier to gas and moisture

Milk, juice and water bottles; trash and retail bags. Soft drink, water and salad dressing bottles; peanut butter and jam jars


High Density Polyethylene

Stiffness, strength, toughness, resistance to moisture, permeability to gas

Milk, juice and water bottles; trash and retail bags


Polyvinyl Chloride

Versatility, clarity, ease of blending, strength, toughness.

Juice bottles; cling films; PVC piping


Low Density Polyethylene

Ease of processing, strength, toughness, flexibility, ease of sealing, barrier to moisture.

Frozen food bags; squeezable bottles, e.g. honey, mustard; cling films; flexible container lids



Strength, toughness, resistance to heat, chemicals, grease and oil, versatile, barrier to moisture.

Reusable microwaveable ware; kitchenware; yogurt containers; margarine tubs; microwaveable disposable take-away containers; disposable cups and plates



Versatility, clarity, easily formed

Egg cartons; packing peanuts; "Styrofoam"; disposable cups, plates, trays and cutlery; disposable take-away containers



Dependent on polymers or combination of polymers

Beverage bottles; baby milk bottles

The Engines of Our Ingenuity is Copyright © 1988-2008 by John H. Lienhard.